The present invention relates to compositions for forming molded articles, and more particularly to compositions including powdered forms of inorganic powders, glass, ceramics, or metals.
A variety of useful molded products having complex shapes and useful mechanical strengths can be made from powdered forms of ceramics, metals, metal oxides, thermoset resins, high melt temperature thermoplastics, and combinations thereof. Examples of these products include aerospace components, biomedical implants, bonded diamond abrasives, cutting tools, turbine blades and other mechanical parts, nozzles subject to continuous contact with abrasives, electronic devices, and superconductors. Forming techniques, such as slip casting, tape casting, extrusion, injection molding, dry pressing or compression molding, generally require the presence of a binder formulation that is mixed with the metal, ceramic, or mixed powder feed. The binder is a temporary vehicle to aid flow during the forming process, for homogeneously packing the powder into the desired shape and then for holding the particles in that shape until the beginning of sintering (German, xe2x80x9cPowder Injection Molding,xe2x80x9d (Metal Powder Industries Federation, Princeton, N.J. 1990); German and Bose, xe2x80x9cInjection Molding of Metals and Ceramics,xe2x80x9d (Metal Powder Industries Federation, Princeton, N.J. 1997)). Sintering or fusing of the powder components is needed, for example, to obtain physical properties for the finished item that are suitable for the conditions of its end use.
One disadvantage with using traditional binders in the shape formation is that the molded product""s physical properties and performance can be impaired by residual amounts of binder or binder decomposition products, by uneven removal of binder or binder decomposition products, or by voids formed by removal of binder or binder decomposition products. (Residual binder is not a problem in the limited circumstances when it is desirable to incorporate binder components into the final form by chemical or interatomic attraction.) Many products made from ceramic powders, metal powders, and blends thereof are used in applications where they are exposed to repeated stresses. Examples of these products include combustion engine parts, valves, rotors, and gear assemblies. Inclusion bodies derived from inadequate removal of binder, or voids resulting from combustion gases during removal, can facilitate cracking and failure of the parts in service. Electrical conductivity is another important performance requirement, for example in electronic parts such as printed circuit boards and superconductors, that can be adversely affected by inadequate binder removal or void formation caused thereby. Therefore, removal of the binder used in shape formation is generally a crucial step in the powder processing technique.
Techniques for the removal of undesirable binders include (1) thermal evaporation; (2) thermal decomposition; (3) chemical transformation to forms useful in the end product; (4) solvent extraction; (5) supercritical extraction; (6) diffusion and absorption of binder constituents to an absorbing surface surrounding the shape or wicking; and (7) depolymerization by thermal means, catalytic means, or a combination thereof. Removal of the binder usually is the slowest step in the powder injection molding process (German, xe2x80x9cSintering Theory and Practice,xe2x80x9d (John Wiley and Sons, New York 1996); German and Bose, xe2x80x9cInjection Molding of Metals and Ceramics,xe2x80x9d (Metal Powder Industries Federation, Princeton, N.J. 1997)). One binding system investigated for providing more rapid removal involves using polyacetals, particularly with injection molding processing, for example, as described in U.S. Pat. No. 5,155,158 to Kim and in WO 91/08993. The use of polyalkylene carbonates for use in such applications is disclosed in European Patent Application EP 0,463,769 A2. In theory, the polyacetal binders xe2x80x9cunzipxe2x80x9d or depolymerize, releasing formaldehyde, when exposed to nitric acid fumes in an incubator. Unfortunately, the use of nitric acid or other oxidants restricts the use of the polyacetal resins to those powders which are not susceptible to undesirable oxidation. Similarly, the polyalkylene carbonate binders xe2x80x9cunzipxe2x80x9d upon reaching a certain decomposition temperature, typically around 200xc2x0 C. Other binder materials include polyoxalate and polymalonate polymers, which also are useful as rheological control agents in paste formulations, as described in U.S. Pat. No. 5,412,062 to Power et al. Polyalkylene carbonates, however, exhibit viscosity behavior that makes flow of the unformed metal/binder, ceramic/binder, or metal/ceramic/binder difficult.
The use of polyhydroxyalkanoates as binders in molding applications is disclosed in WO 99/05209. The approach describes the use of polymers recovered directly from a fermentation process without further modification; typically these polymers will have molecular weights measured by HPLC in the order of 100,000 to in excess of 1,000,000.
Many of the characteristics of materials and compositions useful as binders are described in Shanefield, xe2x80x9cOrganic Additives and Ceramic Processing,xe2x80x9d (Kluwer Academic Publishers, Boston 1996). Desirable features include (1) easy burnout, (2) strong adhesion to the powder and good cohesive strength, (3) solubility in fluidizing liquid, and (4) low cost. The binder material must be suitable for a variety of process conditions, since, for example, many powders must avoid exposure to air or water, or may require exposure to reducing gases or vacuum conditions, during processing.
All of the patents, patent applications, and publications mentioned throughout this application are incorporated in their entirety by reference herein and form a part of the present application.
It is therefore an object of this invention to provide molding compositions having improved binder removal characteristics.
It is another object of this invention to provide molding compositions suitable for use in a wide range of processing conditions.
Molding compositions including polyhydroxyalkanoates are provided. The use of polyhydroxyalkanoates as a binder in molding compositions provides improved binder removal in the finished molded product, and offers a wide range of physical properties suitable for use in a variety of processing conditions. The polyhydroxyalkanoate, in the present invention, preferably has a molecular weight that is less than about 70,000, and more preferably is from about 500 to 20,000, and most preferably, from about 500 to about 10,000. Other molecular weight ranges include from about 1,000 to 5,000, and from about 1,000 to about 3,000.The composition preferably includes a powdered material, such as a metal powder, ceramic powder, or blend, admixed with a polyhydroxyalkanoate binder. The compositions are useful in powder processing techniques, such as injection molding, compression molding, slip casting, tape casting, or extrusion. The compositions containing the low molecular weight polyhydroxyalkanoate preferably exhibit improved green strength and improved handling of green strength.
The present invention further relates to thermally decomposable polyhydroxyalkanoates having at least one of the terminal end groups, and preferably both of the terminal groups selected from:
a) xe2x80x94COxe2x80x94CHxe2x95x90CR9R10;
b) xe2x80x94OR11;
c) xe2x80x94COOR12,
d) xe2x80x94COR13; or
e) xe2x80x94Oxe2x88x92M+
wherein R9, R10, R11, R12, or R13 which are the same or different, represents saturated or unsaturated hydrocarbon radicals, halo- or hydroxy-substituted radicals, hydroxy radicals, nitrogen-substituted radicals, oxygen-substituted radicals, or a hydrogen atom, with the proviso that R11 is not a hydrogen atom. M+is a counter ion. Preferably, both of the terminal end groups of the PHA are selected from one of the above-described terminal end groups a)-e) wherein the terminal end groups can be the same or different. Alternatively, the PHA can have one of the above-described terminal end groups a)-e) and the other end group can end with hydrogen or xe2x80x94OH. Preferably, when the terminal end group is xe2x80x94OR11, or xe2x80x94COR13, the other end group is not xe2x80x94OH. These thermally decomposable polyhydroxyalkanoates can have any molecular weight such as from about 500 to about 1.5 million. More preferably, the molecular weight is from about 500 to about 100,000, more preferably from about 500 to less than 70,000, such as from about 500 to about 10,000. These polyhydroxyalkanoates can be polymers, oligomers, or monomers.
The present invention further relates to a method to make thermally decomposable polyhydroxyalkanoates having low molecular weights which involves the hydrolysis or thermolysis of a polyhydroxyalkanoate under such conditions that will lead to a low molecular weight thermally decomposable polyhydroxyalkanoate or an oligomer thereof, or a monomer thereof.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.